1. Field of the Invention
The present invention relates to a half mirror which transmits light from the back of a mirror body and which is used as rearview mirrors provided at the interior and the exterior of a vehicle.
2. Description of the Related Art
Usually, rearview mirrors are provided at the interior and the exterior of a vehicle compartment. A mirror of this type includes a mirror body. The mirror body includes a plate-configured glass substrate, on which a metal reflecting film is formed. The glass substrate is secured to a mirror holder and is housed in a case, such as a visor.
The mirror holder is formed in a dish-shape, that opens at one end in the depth direction thereof. The mirror body is housed within the mirror holder such that the reflecting film on the glass substrate faces the opening end of the mirror holder. The case, such as a visor, is formed in a dish-shape or a bowl-shape. The mirror holder is housed within the case with the opening end thereof facing the opening end of the case. The mirror holder is fixed at a predetermined position of a vehicle via the casing.
Recently, these inner and outer rearview mirrors have been studied for use not only for providing a rearview of a vehicle but also as a kind of indicator, for example, as a direction indicator when the vehicle turns, or as a speed indicator.
When a mirror is also used as an indicator, a mirror known as a xe2x80x9chalf mirror,xe2x80x9d which transmits light from the back of the reflecting film (i.e., from the glass substrate side thereof), is used as a mirror body. In such a mirror, indicating devices like a turn signal, a character display panel for indicating speed, or the like are disposed at the back of the glass substrate (i.e., on the side of the glass substrate opposite to the side where the reflecting film is formed). When the indicating device lights up, characters and marks are transmitted through the glass substrate and the reflecting film so as to enable an occupant of the vehicle or an occupant of another vehicle approaching from behind (i.e., one who observes the mirror from the opening side of the mirror holder) to observe these characters and marks.
The structure of a mirror body 122 used in an aforementioned half mirror 120 for a vehicle is schematically shown in a cross-sectional view in FIG. 8. As shown in FIG. 8, a silicon (Si) thin film having a thickness T4 of about 20 nm is usually used as a reflecting film 126 provided on a surface of a glass substrate 124 which is a component of a mirror body 122. A protective film 128 is provided at the front of the reflecting film 126. The protective film 128 consists of a silicon dioxide (SiO2) film 130 having the thickness T5 of about 20 nm, which is formed on the reflecting film 126, and a titanium dioxide (TiO2) film 132 having the thickness T6 of about 15 nm, which is formed on the silicon dioxide film 130. The protective film 128 prevents the reflecting film 126 from deterioration, damage that may otherwise be caused to the reflecting film 126 when the mirror body 122 is mounted on the mirror holder or when the above-described indicating device (not shown) is mounted at the back of the mirror body 122, and the like.
Accordingly, three thin films, namely, the reflecting film 126 and the two protective films 128, are provided on the glass substrate 124 of the aforementioned mirror body 122.
In the above-described mirror body 122, the overall reflectance of the mirror body 122 is significantly affected by even slight variations in the thickness of each thin film, namely, the reflecting film 126, the silicon dioxide film 130, and the titanium dioxide film 132. Accordingly, to ensure the reflectance of a predetermined standard, the accuracy in providing a particular film thickness for the reflecting film 126, the silicon dioxide film 130, and the titanium dioxide film 132 respectively, must be strictly controlled. However, because such rigorous control of accuracy in film thickness is extremely difficult, it has become a major reason for increased manufacturing costs.
In view of the aforementioned facts, it is an object of the present invention to provide a half mirror which includes a protective film, for protecting a reflecting film and which has stable quality and does not require any strict quality (film thickness) control, thereby resulting in lower manufacturing costs.
A half mirror relating to a first aspect of the present invention comprises a substrate that at least transmits light having predetermined wavelengths; a reflecting film, comprising substantially silicon of a thickness of 20 to 45 nm and being integrally formed on a surface of the substrate, for reflecting light that is made incident upon the reflecting film from the substrate side thereof and transmitting, toward the substrate side of the reflecting film at a fixed ratio, light made incident upon the reflecting film from the side opposite to the substrate side; and a protective film, which is made of a translucent material that at least transmits light having particular wavelengths, formed on the surface of the reflecting film opposite to the surface on which the substrate is formed.
According to the thus structured half mirror, the light that is made incident upon the reflecting film from the side of the substrate opposite to the surface where the reflecting film is formed (hereinafter, referred to as the front of the substrate) is reflected by the reflecting film. On the other hand, the light that is made incident upon the reflecting film from the side of the protective film opposite to the surface where the reflecting film is formed (hereinafter, referred to as the back of the protective film) is transmitted through the reflecting film, and then passes through the substrate. Accordingly, the light that is made incident upon the reflecting film from the front of the substrate and is reflected at the reflecting film, and the light that is made incident upon the reflecting film from the back of the protective film, can both be observed from the substrate side of the mirror.
In the half mirror, the reflecting film is formed at the back of the substrate. Accordingly, by accommodating the substrate in, for example, a housing (casing) such as a mirror holder having a base, the reflecting film can be protected against any foreign substances which may otherwise contact or attach to the reflecting film. In addition, a protective layer made of a translucent material is formed on the reflecting film. This structure makes it impossible for any foreign substances to directly contact the reflecting film when, for example, the reflecting film is accommodated in aforementioned housing or the like. Accordingly, the reflecting film can be protected from deterioration or damages that may be caused thereto.
In the half mirror of the present invention, because the reflecting film is provided at the back of the substrate and the protective film is provided at the back of the reflecting film, light is made incident upon the reflecting film from the substrate side thereof is basically not affected by the protective film. Accordingly, so long as the thickness of the reflecting film is accurate, even if there is any inaccuracy with respect to the thickness of the protective film, the reflectance of the light, that is made incident upon the reflecting film from the substrate side thereof and is reflected by the reflecting film, is not affected. As a result, a predetermined reflectance can be ensured. As described above, the protective film can be produced with less rigorous thickness control, thereby reducing the manufacturing costs.
Note that, in the present invention, it suffices that the substrate and the protective film can at least transmit light having predetermined wavelengths. That is, the substrate and the protective film may be transparent and transmit light having any wavelengths, or, alternatively, they may transmit or restrict only the light having particular wavelengths.
Further, it is preferable that the half mirror of the present invention further includes a light source which emits light of predetermined wavelengths, the light being transmittable through the protective film and the reflecting film, and the light source being provided at a side of the protective film opposite to the side at which the reflecting film is provided.
In the half mirror having the above structure, a light source, which emits light of predetermined wavelengths, is provided at the back side of the protective film. The light emitted from the light source is transmitted through the protective film and the reflecting film. The light emitted from the light source can therefore be observed from the front of the substrate.
In the present half mirror, the light source is disposed at the back side of the protective film. Since the protective film is interposed between the reflecting film and the light source, the light source cannot contact the reflecting film though it may contact the protective film. Accordingly, when the half mirror is assembled, the light source does not cause any damage to the reflecting film, thus preventing deterioration of the reflecting film.
In addition, it is preferable that the half mirror of the present invention further includes a heating device at a side of the protective film opposite to the side at which the reflecting film is provided.
In the half mirror having the above structure, a heating device is provided at the back side of the protective film. When the heat generated by the heating device is transmitted to the substrate via the protective film and the reflecting film, and the substrate is heated, water droplet or the like attaching to the surface of the substrate, i.e., the surface of the substrate opposite to the surface on which the reflecting film is formed, evaporates. The surface of the substrate can therefore be defogged, thereby ensuring good visibility.
In the present half mirror, the heating device is disposed at the back side of the protective film. Since the protective film is interposed between the reflecting film and the heating device, the heating device cannot contact the reflecting film though it may contact the protective film. Accordingly, when the half mirror is assembled, the heat source does not cause any damage to the reflecting film, thus preventing deterioration of the reflecting film.
A method for manufacturing a half mirror relating to a second aspect of the present invention comprises the steps of; a) forming a reflecting film comprising substantially silicon of a thickness of 20 to 45 nm and disposing said reflecting film integrally on a surface of the substrate, for reflecting light that is made incident upon the reflecting film from the substrate side thereof and transmitting, toward the substrate side of the reflecting film at a fixed ratio, light made incident upon the reflecting film from the side opposite to the substrate side; and b) forming a protective film, which is made of a translucent material that at least transmit light having particular wavelengths and is formed on the surface of the reflecting film opposite to the surface on which the substrate is formed.
It is preferable that the method for manufacturing a half mirror further includes a step of forming a primary protective film that comprises substantially silicon dioxide and interposed between the reflecting film and the protective film, the forming step is being carried out directly after the forming step of the reflecting film.
In the method for manufacturing a half mirror, it is preferable that the step of forming the primary protective film is substantially the same as the process of forming the reflecting film.
According to the half mirror obtained in the above method, a primary protective film, made substantially of silicon dioxide, is interposed between the protective film and the reflecting film. The primary protective film can be obtained in a manufacturing process which is substantially the same as that of the reflecting film, directly after the reflecting film is formed. That is, the primary protective film can be made without moving the substrate after the reflecting film is formed on the substrate. As described above, the protective film is formed after the primary protective film is formed. Even if any foreign substances or the like should come in contact with the substrate when the substrate is transported from a device for making the primary protective film to a device for making the protective film, the reflecting film is protected by the primary protective film against these foreign substances which may otherwise directly contact the reflecting film.